Magnetic resonance imaging (MRI) is a medical imaging technology that is used to visualize detailed internal structures inside a patient's body. MRI machines use the principle of nuclear magnetic resonance to image tissues in a patient's body. First, a strong static magnetic field is used to align the magnetization of hydrogen nuclei (protons) in the body and the strength of this field establishes a resonance frequency of the aligned protons known as the Larmor frequency. A radio frequency (RF) electromagnetic field can then be applied to alter the alignment of the magnetization. By applying the RF electromagnetic field at the Larmor frequency, energy can be efficiently transferred to the aligned protons, changing the way in which they spin. Once the RF electromagnetic field is removed, the protons return to their initial spin state, releasing energy which is then interpreted spatially for the purposes of imaging.
MRI machines are especially good at contrasting the different soft tissues in a patient's body and are therefore very useful in imaging the brain, muscles, etc.
There are a number of different types of MRI machines. Traditional MRI machines operate at static magnetic field strengths that produce Larmor frequencies in the range of tens of Megahertz (MHz). These types of MRI machines operate on the principle of near-field coupling with the detector being placed as close as possible to the patient in the MRI machine and create stationary (i.e. nonpropagating) RF fields. Typically, these types of MRI machines use static magnetic fields having a field strength of 1.5 T which results in a Larmor wavelength of approximately 5 m.
More recently high-field (HF) MRI machines have been used that use higher frequencies and result in higher signal- and contrast-to-noise ratios, allowing for higher-resolution imaging than what can be accomplished using traditional MRI machines. Whereas traditional MRI machines operate at field strengths that produce Larmor frequencies in the range of tens of Megahertz (MHz), HF MRI uses magnetic field strengths that are higher than those of traditional MRI, resulting in Larmor frequencies in the range of hundreds of MHz.
In both traditional MRI machines and HF MRI machines, imaging is accomplished by using transmit/detect coils that generate/detect the required RF fields. The problem with this method is that these coils must be placed very near to the patient being imaged. Typically, the coils are placed around the inside of the bore of the MRI machine so that these coils are adjacent to and surrounding the patient. This closeness of the coils and the confined space the coils create can make patients uncomfortable. Recently a new type of MRI technology has been developed called travelling wave (TW) MM that addresses some of these issues. TW MRI machines use propagating electromagnetic waves passing through the bore of the TW MRI to obtain the images of the patient. Rather than having to place transmit/detect coils beside the body of a patient, TW MRI uses waves that are excited by RF antennas placed at either end of the TW MRI bore. This allows all of the hardware for generating and detecting these waves to be placed away from where the patient is when the TW MRI is in operation.
In TW MRI machines, the bore of the MRI acts a cylindrical waveguide for the electromagnetic waves propagating through them. The electromagnetic waves propagating through a cylindrical waveguide may be classified into modes, such as the Transverse Electric (TE) modes, and by mode indices (e.g. 11), which identify the way in which the modal fields vary in the transverse waveguide plane. These electromagnetic waves propagate through the bore of the MRI using the conductive inner surface of the bore. Like a waveguide, the TW MRI bore has a cutoff frequency for propagating waves, and because of the size of bore required to accommodate the body of the patient, this cutoff frequency is in the order of several hundred MHz. For example, a typical MRI bore may be 58 cm in diameter and have a natural frequency cutoff of the TE11 mode of approximately 300 MHz. This natural cutoff frequency of the MRI bore prevents waves having a frequency below the natural cutoff frequency from propagating through the MRI bore. This requires TW MRI bores to have larger magnets and create strong enough magnetic fields that the generated waves have a frequency greater than the natural cutoff frequency of the MRI bore. It also prevents more traditional MRI machines from being used as TW MRI machines because they do not possess strong enough magnets to generate waves that have a frequency greater than the natural cutoff frequency of the MRI bore.